Before a catalyst is selected for use in a commercial application, for example hydrocarbon reactions in petroleum refining, a great number of catalysts may be examined for use in the envisioned application. A large number of newly synthesized catalytic compositions may be considered as candidates. It then becomes important to evaluate each of the potential catalysts to determine the formulations that are the most successful in catalyzing the reaction of interest under a given set of reaction conditions.
Two key characteristics of a catalyst that are determinative of its success are the activity of that catalyst and the selectivity of the catalyst. The term activity refers to the rate of conversion of reactants by a given amount of catalyst under specified conditions, and the term selectivity refers to the degree to which a given catalyst favors one reaction compared with another possible reaction, see, McGraw-Hill Concise Encyclopedia of Science and Technology, Parker, S. B., Ed. in Chief; McGraw-Hill: New York, 1984; p. 8.
The traditional approach to evaluating the activity and selectivity of new catalysts is a sequential one. When using a micro-reactor or pilot plant, each catalyst is independently tested at a set of specified conditions. Upon completion of the test at each of the set of specified conditions, the current catalyst is removed from the micro-reactor or pilot plant and the next catalyst is loaded. The testing is repeated on the freshly loaded catalyst. The process is repeated sequentially for each of the catalyst formulations. Overall, the process of testing all new catalyst formulations is a lengthy process at best.
Combinatorial chemistry deals mainly with the synthesis of new compounds. For example, U.S. Pat. No. 5,612,002 B1 and U.S. Pat. No. 5,766,556 B1 teach an apparatus and a method for simultaneous synthesis of multiple compounds. Akporiaye, D. E.; Dahl, I. M.; Karlsson, A.; Wendelbo, R. Angew Chem. Int. Ed. 1998, 37, 9-611 disclose a combinatorial approach to the hydrothermal synthesis of zeolites, see also WO 98/36826.
Combinatorial methods present the possibility of substantially increasing the efficiency of catalyst evaluation. Recently, efforts have been made to use combinatorial methods to increase the efficiency and decrease the time necessary for thorough catalyst testing. For example, WO 97/32208-A1 teaches placing different catalysts in a multi-cell holder with the heat absorbed or liberated in each cell being measured to determine the extent of each reaction. Thermal imaging has also been used; see Holzwarth, A.; Schmodt, H.; Maier, W. F. Angew. Chem. Int. Ed., 1998, 37,-47, and Bein, T. Angew. Chem. Int. Ed., 1999, 3-3. Measuring the heat absorption or liberation and thermal imaging may provide semi-quantitative data regarding activity of the catalyst in question, but they provide no information about selectivity.
Some attempts to acquire information as to the reaction products in rapid-throughput catalyst testing are described in Senkan, S. M. Nature, July 1998, 4(23), 3-353, where laser-induced resonance-enhanced multiphoton ionization is used to analyze a gas flow from each of the fixed catalyst sites. Similarly, Cong, P.; Doolen, R. D.; Fan, Q.; Giaquinta, D. M.; Guan, S.; McFarland, E. W.; Poojary, D. M.; Self, K.; Turner, H. W.; Weinberg, W. H. Angew Chem. Int. Ed. 1999, 4-8 teach using a probe with concentric tubing for gas delivery/removal and sampling. Only the fixed bed of catalyst being tested is exposed to the reactant stream, with the excess reactants being removed via vacuum. The single fixed bed of catalyst being tested is heated and the gas mixture directly above the catalyst is sampled and sent to a mass spectrometer.
Attempts have been made to apply combinatorial chemistry to evaluate the activity of catalysts. Some applications have focused on determining the relative activity of catalysts in a library; see Klien, J.; Lehmann, C. W.; Schmidt, H.; Maier, W. F. Angew Chem. Int. Ed. 1998, 37, 39-3372; Taylor, S. J.; Morken, J. P. Science, April 1998, 0(10), 7-270; and WO 99/34206-A1. Some applications have broadened the information sought to include the selectivity of catalysts. WO 99/19724-A1 discloses screening for activities and selectivities of catalyst libraries having addressable test sites by contacting potential catalysts at the test sites with reactant streams forming product plumes. The product plumes are screened by passing a radiation beam of an energy level to promote photoions and photoelectrons which are detected by microelectrode collection. WO 98/07026-A1 discloses miniaturized reactors where the reaction mixture is analyzed during the reaction time using spectroscopic analysis.
In order to determine the activity and selectivity of multiple catalysts, arrays of reactors have been designed to simultaneously examine multiple catalysts using the above mentioned analysis techniques. For example, EP 1108467 A2 teaches reactors with removable sections to allow easy introduction of catalyst to the reactor bed. The reactors are sealed using o-rings to allow quick connection of the reactor parts and also provide a reliable seal between the reactor parts and between each reactor and its environment.
Many reactors available currently are designed for the situation where the feed streams are all of the same phase, for example two feed components that are both gases. Many process technologies and chemistries require higher-pressure gas-phase catalysis, in which a liquid feedstock is vaporized before contacting the catalyst. This may become challenging due to the fact that many seals used for combinatorial arrays have a temperature limitation that is below the bubble point of many reactor inlet compositional mixtures. For example, the long-term temperature limitation on a typical O-ring seal is about 170° C., while the bubble point of C6 to C9 hydrocarbons, for example toluene, at operating pressures of about 300 psig (2172 kPa) to about 450 psig (3220 kPa) are between about 180° C. and about 240° C. at a hydrogen to toluene molar ratio between about 1 and about 3.
U.S. Pat. No. 5,453,526 B1 teaches a catalytic reactor where liquid media can be continuously introduced, evaporated, and fed to a catalytic reaction. U.S. Pat. No. 3,359,074 teaches a polycondensation system of a single vertically extending column which is transversely partitioned to define, in descending order, a reaction chamber, an evaporator chamber, and a finishing chamber. Two articles, Bej K. S.; Rao, M. S. Ind. Eng. Chem. Res., 1991 30 (8), 1819-1832, and Eliezer K. F.; Bhinde, M.; Houalla, M.; Broderick, D.; Gates, B. C.; Katzer, J. R.; Olson, J. H. Ind. Eng. Chem. Fundam., 1977, 16 (3), 380-385 show where additional particles are used to aid in flow distribution before a feed is contacted with a catalyst. What is needed is an evaporator that can be integrated into a process vessel, that accommodates a liquid feed so that the seals will not be compromised during operation of the process vessel, while providing for the feed to be in a vapor phase during reaction.
However, evaporators in general have some inherent problems associated with their operation. One problem associated with evaporators in general is non-uniform mixing of a liquid feed and a gas feed. Non-uniform mixing may occur when both a gas and a liquid are introduced to an evaporator through a common inlet. The dual feed of liquid and gas causes alternating regions of gas entrainment and liquid pulsation being introduced to an evaporator, and therefore regions of low concentration of the vaporized species followed by regions of high concentration of the vaporized species being sent to a reactor bed.
Another problem associated with non-uniform vaporization occurs mainly because of a non-uniform flow of liquid into an evaporator. In the case of slower moving flow, a liquid issuing from an orifice into an evaporator can form droplets that detach at a regular periodicity because of the fluid dynamics of the liquid. The periodic formation and detachment of droplets leads to non-uniform vaporization within the evaporator.
What is needed is an evaporator for use in a process vessel that overcomes the problems of non-uniform mixing and non-uniform vaporization associated with evaporators in general.